It is now almost a matter of routine to synthesize a single defined peptide sequence using the Merrifield method to "grow" peptide chains attached to solid supports. The process of synthesizing these individual peptides has, in fact, been automated, and commercially available equipment can be used to synthesize routinely peptides of twenty or more amino acids in length. To obtain peptides of arbitrary length, the resulting peptides can further be ligated with each other by using appropriate protective groups on the side chains and by employing techniques permitting the removal of the synthesized peptides from the solid supports without deprotecting them. Thus, the synthesis of individual peptides of arbitrary length is known in the art.
However routine the synthesis of individual peptides may be, it is necessarily laborious. Therefore, in the many cases where it is not previously known which of a multiplicity of peptides is, in fact, the preparation desired, while theoretically it is possible to synthesize all possible candidates and test them with whatever assay is relevant (immunoreactivity with a specific antibody, interaction with a specific receptor, particular biological activity, etc.), to do so using the foregoing method would be comparable to the generation of the proverbial Shakespeare play by the infinite number of monkeys with their infinite number of typewriters. In general, the search for suitable peptides for a particular purpose has been conducted only in cases where there is some prior knowledge of the most probable successful sequence. Therefore, methods to systematize the synthesis of a multiplicity of peptides for testing in assay systems would have great benefits in efficiency and economy, and permit extrapolation to cases where nothing is known about the desired sequence.
Two such methods have so far been disclosed. One of them, that of Houghten, R. A., Proc Natl Acad Sci USA (1985) 82:5131-5135, is a modification of the above Merrifield method using individual polyethylene bags. In the general Merrifield method, the C-terminal amino acid of the desired peptide is attached to a solid support, and the peptide chain is formed by sequentially adding amino acid residues, thus extending the chain to the N-terminus. The additions are carried out in sequential steps involving deprotection, attachment of the next amino acid residue in protected form, deprotection of the peptide, attachment of the next protected residue, and so forth.
In the Houghten method, individual polyethylene bags containing C-terminal amino acids bound to solid support can be mixed and matched through the sequential attachment procedures so that, for example, twenty bags containing different C-terminal residues attached to the support can be simultaneously deprotected and treated with the same protected amino acid residue to be next attached, and then recovered and treated uniformly or differently, as desired. The resultant of this is a series of polyethylene bags each containing a different peptide sequence. These sequences can then be recovered and individually biologically tested.
An alternative method has been devised by Geysen, H. M., et al, Proc Natl Acad Sci USA (1984) 81:3998-4002. See also W086/06487 and W086/00991. This method is a modification of the Merrifield system wherein the C-terminal amino acid residues are bound to solid supports in the form of polyethylene pins and the pins treated individually or collectively in sequence to attach the remaining amino acid residues. Without removing the peptides from support, these peptides can then efficiently be effectively individually assessed for the desired activity, in the case of the Geysen work, interaction with a given antibody. The Geysen procedure results in considerable gains in efficiency of both the synthesis and testing procedures, while nevertheless producing individual different peptides. It is workable, however, only in instances where the assay can be practically conducted on the pin-type supports used. If solution assay methods are required, the Geysen approach would be impractical.
Thus, there remains a need for an efficient method to synthesize a multiplicity of peptides and to select and analyze these peptides for those which have a particular desired biological property. The present invention offers such an alternative by utilizing synthesis of mixtures as well as providing a means to isolate and analyze those members or families of members of the mixture which have the desired property.